Trabecular meshwork. Multilayered network of fenestrated
lamellae and endothelial cells draining aqueous into Canal
of Schlemm which may visible when full of blood (e.g. in
hypotony or when excess force applied to sclera during gonioscopy.
Most of the drainage occurs via the posterior, more pigmented,
portion of the trabecular meshwork. There are variations in
colour but usually grey with varying degrees of pigmentation.

Schwalbe's line. Delineates the anterior edge of the
trabecular zone and represents the termination of Descemet's
membrane. Very fine glossy white line.

Much of the anterior chamber is readily visible with the slit
lamp biomicroscope. However the approach to the trabecular region
is obscured from view in all eyes except some rare exceptions
for two reasons:

the overlap of the the sclera obscuring the frontal view

oscuration of a lateral view because of total internal reflection
within the cornea, see (Figure 2)

Figure 2. Light returning from the angle is totally internally
reflected within the cornea

Because of the reasons outlined above, in order to view the trabecular
region the effect of the corneal curvature must be neutralised.
This can be

produced in two ways.

(a) Direct gonioscopy - the anterior curve of the contact
lens (goniolens)is such that the critical
angle is not reached and the light rays are refracted at the
contact lens/air interface, see Figure 3.

Figure 3 Goniolens

(b) Indirect gonioscopy - light rays are reflected by
a mirror in the contact lens (gonioprism) and leave
the lens at nearly a right angle to the contact lens-air interface,
see Figure 4.

Figure 4 Goniolens

Gonioscopic grading of the angle

There are a number of grading systems.

Schaffer's method

Simplistic but useful.

Estimation of the angle which the iris makes with the ciliary
body/trabeculum.

Corelates well with Van Herrick. (see Table 1)

Does not take into account position of iris insertion
or the pupillary block scenario